Method and device for magnetizing annular discs in radial direction

ABSTRACT

A method of radially magnetizing an annular disc includes covering a part of at least one flat face of the disc with a ring of soft-magnetic material having a diameter not greater than the inner diameter of the disc and having an outer diameter less than that of the disc and magnetic flux is passed radially through the disc between the outer and inner circumferential areas. The ring is removed from the disc and magnetic flux is again passed radially through the disc. Device for carrying out the method includes a magnetic coil for generating a magnetic field. The coil has a core for conducting the flux of the field. A mandrel of magnetically permeable material mechanically engages the core and accommodates the disc and ring in the magnetic field. The core has a peripheral surface of which a portion is spaced from the mandrel. A yoke of magnetically permeable material is provided and magnetically communicates with the core of the surface portion of the latter spaced from the mandrel. The yoke conducts the flux to the outer peripheral surface of the annular disc to be magnetized.

United States Patent [72] Inventor Kathe Muller Spindelmuhler Weg 30,I000 Berlin 45, Germany [2i Appl. No. 852,299

[22] Filed Aug. 22, 1969 [45] Patented June 15, I971 [32] Priority Aug.22, 1968 [3 3] Switurland [54] METHOD AND DEVICE FOR MAGNETIZING ANNULARDISCS IN RADIAL DIRECTION 11 Claims, 2 Drawing Figs.

[52] US. Cl. 335/284, 335/302 [5i] Int. (I n01: 13/00 [50] Field ofSearch 335/284, 302, 210

[56] References Cited UNITED STATES PATENTS 2,248,272 7/1941 .lurak335/284 2,272,766 2/1942 Carson et a!.. 335/284 3,237,059 2/1966 Meyerer335/210 3,375,400 3/1968 Schrumpf 335/210 X Attorneys-Curt M. Avery,Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick ABSTRACT: A- method of radially magnetizing anannular disc includes covering a part of at least one flat face of thedisc with a ring of soft-magnetic material having a diameter not greaterthan the inner diameter of the disc and having an outer diameter lessthan that of the disc and magnetic flux is passed radially through thedisc between the outer and inner circumferential areas. The ring isremoved from the disc and magneticflux is again passed radially throughthe disc.

Device for carrying out the method includes a magnetic coil forgenerating a magnetic field. The coil has a core for conducting the fluxof the field. A mandrel of magnetically permeable material mechanicallyengages the core and accommodates the disc and ring in the magneticfield. The core has a peripheral surface of which a portion is spacedfrom the mandrel. A yoke of magnetically permeable material is providedand magnetically communicates with the core of the surface portion ofthe latter spaced from the mandrel. The yoke conducts the flux to theouter peripheral surface of the annular disc to be magnetized.

We II PATENTED JUN] 519m Inventor METHOD AND DEVICE FOR MAGNETIZINGANNULAR DISCS IN RADIAL DIRECTION My invention relates to a method anddevice for magnetizing annular discs in radial direction.

The annular discs which are magnetized according to the method of theinvention by means of the device of the invention are used primarily asfield-generating components of a permanently magnetized lens incooperation with aperture discs magnetized in rotational symmetry. It isknown, for example, from British Patent 522,377, to use such annulardiscs, magnetized in radial direction, as electron-optical lenses whosecentral opening serves as the passage for the electron beam to be actedupon. Lenses constructed in this manner can also be used in otherparticle beam devices, such as in diffraction devices or ionmicroscopes.

Tests and experiments have demonstrated that this magnetization ofannular discs in radial direction creates difficulties, at least whenthe material should be magnetized completely, Because the material crosssection available to the magnetic flux is considerably smaller in theregion of the inner diameter of the disc than in the region of the outerdiameter thereof. It has been found that insofar as the annular disc issubjected, in the absence of other steps, to a magnetizing field, themagnetic flux passing though the disc is not high enough to secure acomplete magnetization of the outer regions of the annular disc which isdue to a premature magnetic saturation of the material in the innerregion of the disc.

It is an object of my invention to provide 'a method and means suitablefor magnetizing annular discs which affords complete magnetization ofall regions of the disc.

According to a feature of the invention, magnetization of an annulardisc is performed from its outer periphery. More specifically, the discis subjected to a magnetic field which acts in the plane of the disc inradial direction between the outer and inner peripheral areas of thedisc. First, at least one of the flat surfaces of the disc is covered bya ring of soft-magnetic material in the region whose limits are definedby a circle having an inner diameter not larger than the inner diameterof the disc and a circle having an outer diameter less than that of thedisc. The elevation of the soft-magnetic ring increases with radialdistance measured inwardly from the outer circle to the inner circle.The annular disc is subsequently subjected to the magnetic field withoutthe soft-magnetic ring,

According to more specific features of the invention, whilemagnetization takes place, saturation of the inner region of the annulardisc is prevented, whereas its outer region becomes magnetized becausemagnetic shunts are provided in parallel with the inner region, so thatthe magnetic flux which passes through the outer region is notrestricted by saturation of the inner region. in order to reliablyobtain a complete magnetization of the material of the inner region ofthe annular disc as well, the disc is subjected in a subsequent step tothe magnetic field without having the soft-magnetic ring disposedthereon.

According to another embodiment of the invention, the magnetization ofthe annular disc is performed in more than two steps by exposing thedisc to the magnetic field; first, by sequentially covering the regionsof at least one of its front faces with several soft-magnetic rings orring pairs having a smaller outer diameter than that of the previouslyused rings or pairs of rings. Thus, several soft-magnetic rings are usedhaving respective outer diameters which become smaller with eachsuccessive method step while the inner ring diameter has the same valuein each instance. This produces a step-by-step magnetization of the discproceeding from the outside inwardly, so that saturation in the innerregion of the disc cannot produce an adverse effect upon themagnetization of the outer region. As a final step, these method stepsare followed by the magnetization of the inner region of the disc in theabsence of the soft-magnetic rings which served as magnetic shunts.

The method of the invention affords the advantage of allowing theannular discs to be magnetized along desired radii, as

well as effecting a rotation-symmetrical magnetization, the latter beingof particular importance where, for example, the discs are used asexcitation elements of permanent-magnetic lenses. To precludeunavoidable tolerances from acting as disturbing deviations from therotational-symmetry of the magnetization, it is preferable for achievingrotation-symmetrical magnetization of the annular discs to rotate thelatter in several positions with and without the cover rings and to subject the same to the magnetic field in these positions.

' The device for performing the method of the invention must beconstructed to supply the magnetizing flux to the inside circumferenceof the discs to be magnetized as well as to the softmagnetic rings andbe able to receive the flux at the outer circumference of the disc orconversely.

A device for performing the method of the invention which fulfills theabove-mentioned criteria has a mandrel upon which the annular discs tobe magnetized are placed together with the rings which are sometimesneeded. The mandrel is made of a material having a low magneticresistivity or high penneability and has longitudinal extensions whichextend.

into the centric openings of the coils that produce the magnetic field.The extensions serve as cores for the coils and have respectiveperipheral surfaces of which a portion is spaced from the mandrel. Thesesurface portions are connected with a magnetic yoke which extends intothe region of the outer circumference of the annular disc.

1n the method of the invention, the member used to hold the annular discto be magnetized as well as the soft-magnetic rings also delivers themagnetic flux to the inner peripheral areas of the disc and rings.

The yoke portion of the magnetic circuit does not necessarily have tobear upon the outer surface of the disc for it to be magnetized, rather,a good magnetizing effect is achieved even when an airgap is presentbetween the magnetic yoke and the annular disc. I have found to be mosteffective a device wherein the yoke is provided with a yoke plate whichsurrounds the outer peripheral area of the annular disc and which has athickness greater than that of the disc, at least on the outside. Inaddition, the plate tapers to lesser thickness in the direction of thedisc.

Hence, while the elevation of the soft-magnetic rings which bear asshunts upon the inner regions of the discs to be magnetized, ispreferably reduced in a radial outward direction, the thickness of theyoke plate diminishes in the opposite radial direction so that its valueat the contact point with the outer peripheral area of the annular disccorresponds approximately to the thickness of the disc. Thisdimensioning of the soft-magnetic rings and the yoke plate was found tobe advantageous in regard to reducing stray flux in the vicinity of theannular disc.

It is possible to provide an embodiment of the device for performing theinvention which involves an asymmetrical construction and produces themagnetic flux used to effect magnetization with one or more windingsarranged only to one side of the annular disc to be magnetized. However,for securing the best feasible homogeneity of magnetization in the discwith respect to rotational symmetry of magnetization as well as withrespect to the center plane of the disc which extends parallel to fiatsurfaces thereof, it is preferable to position the disc in the deviceturned once or several times by and to subject the disc to magnetizationin each such position.

This expedient becomes unnecessary if each side of the mandrel isprovided with a magnetic coil and core which drive aligned magneticfluxes via a common yoke through the annular disc as well as the ringswhich are sometimes used. In the mandrel, the coils produce mutuallyopposed magnetic fluxes which extend in the same directions in theannular disc to be magnetized.

As already discussed, it is desirable for obtaining arotationsymmetrical magnetization to rotate the annular discs to severalpositions and to subject them to the magnetic field. A feature of themagnetizing device of the invention is that the mandrel is rotatableabout its longitudinal axis during the magnetization process. Rotationof the mandrel can be effected manually as well as by a relatively slowrotating electric motor.

The magnetizing device is preferably subdivided at a place suitable forinserting and removing the annular disc and the rings. Also the mandrelwith the discs and the rings when used are rotatably positioned in thedirection of the longitudinal axis of the mandrel to facilitate theremoval of the annular discs from the vicinity of the yoke plate.

The invention will be further elucidated with reference to theaccompanying drawing showing by way of example an embodiment of amagnetizing device according to the invention.

FIG. 1 is a sectional view of a device for magnetizing annular discs inaccordance with the method of the invention.

FIG. 2 is a sectional view. of the magnetizing device when viewed fromlines Il-II in FIG. 1.

In FIG. 1 annular disc 1 to be magnetized is held by the mandrel 2 madeof a material having low magnetic resistivity. The mandrel 2 buttsagainst the magnetic core 3 with its right end, the latter extendinginto coil 4. Coil 4 produces a north pole N at the end of the magnetcore 3 which faces the mandrel 2.

The left end of the mandrel 2 extends into pole piece 5 at magnet core6, the latter extending into coil 7. This arrangement establishes aconnection between the mandrel 2 and the magnet core 6 which isdetachable along the interface 5a by moving the magnetic core 3 to theright.

The magnetic circuits close over the yoke 8 which is common to coils 4and 7 as well as over yoke plate 9 which bears upon the outer peripheralarea of the annular disc 1 to be magnetized.

Referring to FIG. 2, the magnetic yoke 8 is essentially comprised of twostrutlike parts 10 and 11. The yoke plate consists of two halves l3, 14divided at the junction interface 12 which lies on a line passingthrough the axis of the mandrel 2. Plate halves 13, 14 are detachablyjoined together by screws or other suitable means after the disc 1 to bemagnetized, is placed in position.

FIGS. 1 and 2 illustrate the disc I after the inner regions thereof havebeen covered by soft-magnetic rings 15, 16 according to a step of themethod of the invention, the rings 15, 16 functioning to preventlimiting the magnetic flux which crosses the disc 1 because ofsaturation within the inner region of the latter.

The positioning of the annular disc 1 and rings l5, l6 and theirsubsequent removal from' the magnetizing device becomes especiallysimple if there is no permanent connection between the mandrel 2 and themagnetic core 3 and if instead, only a bearing surface is provided asillustrated. The annular disc 1 and when required, rings l5, 16 arefirst threaded on the mandrel outside the device. The assembly thusformed is placed in the magnetizing device while the yoke plate 9 isopened, for example, by removing plate half 14. A good contact betweenmandrel 2 and core 3 is established by shifting the latter. The platehalf 14 of the yoke plate 9 is then screwed or otherwise secured toplate half 13.

Even if the magnetic return circuit is not rotationally symmetric, auniformity in the magnetizing flux is still effected along the yokeplate 9, since the circuit has only the two strutlike parts 10, 11, sothat a rotation-symmetrical magnetization of the annular disc isobtained. Stray flux in the vicinity of the annular disc 1 is held at aminimum because of the tapered construction of the yoke plate 9indicated by reference numerals 17 and 18. At 17 and 18, the thicknessof the plate 9, is conically reduced to correspond approximately to thethickness of the disc I. The thickness of the soft-matnetic ringslikewise decreased with radial distance to keep stray flux at a minimum.

When the magnetization of the annular disc 1 is not effected inrotational symmetry, but only along specific radii, the mandrel 2 andthe yoke plate 9 are constructed so as not to bear upon the disc I alongthe entire inner and outer circumferential areas respectively of thelatter, but only in the regions corresponding to the desired radii. Theannular disc does not necessarily have to be an annular disc, but canhave, for example, outer boundary areas, which are planar such as apolygon.

Upon study of this disclosure it will be obvious to those skilled in theart that my invention permits of a great variety of modifications in amanner analogous to the above-mentioned magnetizing method and device.For example, the mandrel and magnetic cores can together constitute asingle member.

I claim:

l. The method of radially magnetizing an annular disc which comprisescovering a part of at least one of the flat faces of the disc with aring of soft-magnetic material having an inner diameter not greater thanthe inner diameter of said disc and having an outer diameter less thanthat of said disc, passing magnetic fiux radially through the discbetween its outer and inner circumferential areas, removing said ringfrom said annular disc and, again passing magnetic flux radially throughthe disc.

2. The method according to claim 1, which comprises sequentiallycovering a part of at least one of the flat surfaces of the annular discwith a plurality of said rings, one at a time, each succeeding ringhaving an outer diameter less than the outer diameter of the precedingring, passing magnetic flux radially through said disc while coveringsaid disc with a first one of said rings, and passing magnetic fluxthrough said disc at least once more with another one of said pluralityof said rings having an outer diameter less than the outer diameter ofthe first of said rings.

3. The method according to claim 2, wherein each of said rings has athickness which increases with radial distance from its outer peripheryto its inner periphery.

4. The method according to claim 1, wherein said disc is magnetized inrotational symmetry, said method comprising rotating said disc with saidring to a plurality of angular positions, passing magnetic flux radiallythrough said disc at a first one of said positions, and passing magneticflux through said disc at least once more at another one of saidplurality of positions, and after removing said ring form said disc,again rotating said disc to a second plurality of angular positions, andpassing magnetic flux through said disc at a first one of said secondplurality of positions, and passing magnetic flux through said disc atleast once more at another one of said second plurality of angularpositions.

. 5. A device for radially magnetizing an annular disc with the aidmandrel, ring means made of soft-magnetic material, said devicecomprising magnetic coil means for generating a magnetic field, saidcoil means having core means for conducting the flux of said field, amandrel of magnetically permeable material for accommodating the discand ring means in said magnetic field, said mandrel mechanicallyengaging said core means, said core means having a peripheral surface ofwhich a portion is spaced from said mandrel, and a yoke of magneticallypermeable material magnetically communicating with said core means atsaid surface portion and being adapted to conduct said flux to the outerperipheral surface of the annular disc to be magnetized.

6. In a device according to claim 5, said yoke having a platesurrounding the outer peripheral surface of the disc, said plate havinga thickness at its outermost region greater than the thickness of thedisc, said thickness decreasing with radial distance measured inwardlytoward the disc.

7. In a device according to claim 5, said core means being two coilcores disposed at the axially opposite ends of said mandrel, each ofsaid cores having a peripheral surface, a portion of which is spacedfrom .said mandrel, said coil means being two coils surrounding saidcores respectively, and said yoke magnetically communicating with eachof said cores at said surface portion thereof for directing said flux insymmetrical distribution through the disc and ring means in the samedirection.

8. In a device according to claim 5, said mandrel and said core meanshaving coupling means for establishing a disconnectable connectiontherebetween and so permit placement and removal of the disc and ringmeans on said mandrel.

9. In a device according to claim 6, said plate consisting of two partsdisconnectable along a junction interface disposed on a line passingthrough the longitudinal axis of said mandrel.

said mandrel having a common longitudinal axis, said mandrel with thedisc and ring means being displaceable along said axis.

1. The method of radially magnetizing an annular disc which comprisescovering a part of at least one of the flat faces of the disc with aring of soft-magnetic material having an inner diameter not greater thanthe inner diameter of said disc and having an outer diameter less thanthat of said disc, passing magnetic flux radially through the discbetween its outer and inner circumferential areas, removing said ringfrom said annular disc and, again passing magnetic flux radially throughthe disc.
 2. The method according to claim 1, which comprisessequentially covering a part of at least one of the flat surfaces of theannular disc with a plurality of said rings, one at a time, eachsucceeding ring having an outer diameter less than the outer diameter ofthe preceding ring, passing magnetic flux radially through said discwhile covering said disc with a first one of said rings, and passingmagnetic flux through said disc at least once more with another one ofsaid plurality of said rings having an outer diameter less than theouter diameter of the first of said rings.
 3. The method according toclaim 2, wherein each of said rings has a thickness which increases withradial distance from its outer periphery to its inner periphery.
 4. Themethod according to claim 1, wherein said disc is magnetized inrotational symmetry, said method comprising rotating said disc with saidring to a plurality of angular positions, passing magnetic flux radiallythrough said disc at a first one of said positions, and passing magneticflux through said disc at least once more at another one of saidplurality of positions, and after removing said ring form said disc,again rotating said disc to a second plurality of angular positions, andpassing magnetic flux through said disc at a first one of said secondplurality of positions, and passing magnetic flux through said disc atleast once more at another one of said second plurality of angularpositions.
 5. A device for radially magnetizing an annular disc with theaid mandrel, ring means made of soft-magnetic material, said devicecomprising magnetic coil means for generating a magnetic field, saidcoil means having core means for conducting the flux of said field, amandrel of magnetically permeable material for accommodating the discand ring means in said magnetic field, said mandrel mechanicallyengaging said core means, said core means having a peripheral surface ofwhich a portion is spaced from said mandrel, and a yoke of magneticallypermeable material magnetically communicating with said core means atsaid surface portion and being adapted to conduct said flux to the outerperipheral surface of the annular disc to be magnetized.
 6. In a deviceaccording to claim 5, said yoke having a plate surrounding the outerperipheral surface of the disc, said plate having a thickness at itsoutermost region greater than the thickness of the disc, said thicknessdecreasing with radial distance measured inwardly toward the disc.
 7. Ina device according to claim 5, said core means being two coil coresdisposed at the axially opposite ends of said mandrel, each of saidcores having a peripheral surface, a portion of which is spaced fromsaid mandrel, said coil means being two coils surrounding said coresrespectively, and said yoke magnetically communicating with each of saidcores at said surface portion thereof for directing said flux insymmetrical distribution through the disc and ring means in the samedirection.
 8. In a device according to claim 5, said mandrel and saidcore means having coupling means for establishing a disconnectableconnection therebetween and so permit placement and removal of the discand ring means on said mandrel.
 9. In a device according to claim 6,said plate consisting of two parts disconnectable along a junctioninterface disposed on a line passing through the longitudinal axis ofsaid mandrel.
 10. In a device according to claim 5, said core meansbeing rotatably disposed in said coil means so as to permit rotation ofsaid mandrel about its longitudinal axis.
 11. In a deice according toclaim 5, said core means and said mandrel having a common longitudinalaxis, said mandrel with the disc and ring means being displaceable alongsaid axis.